1. Field of the Invention
The present invention relates to a pixel circuit (referred to also as a pixel) including an electro-optical element (referred to also as a display element and a light-emitting element), a display having a pixel array part in which the pixel circuits are arranged in a matrix, and a method for driving the pixel circuit. More specifically, the invention relates to a pixel circuit having as a display element an electro-optical element of which luminance changes depending on the magnitude of a drive signal, an active-matrix display in which each pixel circuit has active elements and driving for displaying is carried out by the active elements on a pixel-by-pixel basis, and a method for driving the pixel circuit.
2. Description of the Related Art
There have been developed displays that employ, as display elements for pixels, electro-optical elements of which luminance changes depending on voltage applied thereto or current flow therethrough. For example, a liquid crystal display element is a representative example of electro-optical elements of which luminance changes depending on voltage applied thereto, and an organic electro luminescence (hereinafter, referred to as organic EL) element (organic light emitting diode (OLED)) is a representative example of electro-optical elements of which luminance changes depending on current flow therethrough. An organic EL display employing the organic EL elements is a so-called self-luminous display that employs self-luminous electro-optical elements as display elements for pixels.
The organic EL element is an electro-optical element employing a phenomenon that an organic thin film emits light when an electric field is applied thereto. The organic EL element is a low power consumption element because it can be driven by comparatively low application voltage (e.g., 10 V or lower). Furthermore, because the organic EL element is a self-luminous element that emits light by itself, it does not need an assistant illumination member such as a backlight, which is required in a liquid crystal display, and thus can easily achieve reduction in the display weight and thickness. Moreover, the response speed of the organic EL element is very high (e.g., several microseconds), and therefore no image lag occurs in displaying of a moving image. Because of these advantages, development of flat self-luminous displays employing the organic EL elements as electro-optical elements is being actively promoted in recent years.
As the driving system of displays including electro-optical elements typified by liquid crystal displays including liquid crystal display elements and organic EL displays including organic EL elements, a simple (passive)-matrix system or active-matrix system can be employed. However, a display of the simple-matrix system involves e.g. a problem that it is difficult to realize a large-size and high-definition display although the configuration thereof is simple.
For that reason, in recent years, development is being actively promoted on displays of the active-matrix system in which a pixel signal supplied to a light-emitting element inside the pixel is controlled by using an active element such as an insulated gate field effect transistor (typically, thin film transistor (TFT)) provided inside the pixel as a switching transistor.
In the active-matrix display, for light emission of an electro-optical element in a pixel circuit, an input image signal supplied via a video signal line is loaded through the switching transistor into a holding capacitor (referred to also as a pixel capacitor) provided for the gate (control input terminal) of a drive transistor, so that a drive signal dependent upon the loaded input image signal is supplied to the electro-optical element.
In the case of a liquid crystal display including liquid crystal display elements as electro-optical elements, the liquid crystal display element is a voltage-driven element and therefore is driven by a voltage signal itself dependent upon the input image signal loaded in the holding capacitor. In contrast, in an organic EL display that employs current-driven elements such as organic EL elements as electro-optical elements, a drive signal (voltage signal) dependent upon the input image signal loaded in the holding capacitor is converted to a current signal by a drive transistor, so that the drive current is supplied to the organic EL element or the like.
In the case of current-driven electro-optical elements typified by organic EL elements, if the drive current value differs, the light emission luminance also differs. Therefore, for light emission with stable luminance, it is vital to supply a stable drive current to an electro-optical element. For example, the driving systems for supplying a drive current to an organic EL element can be roughly classified into a constant-current driving system and a constant-voltage driving system (publicly-known documents thereof are not shown here, because these systems are well-known techniques).
The voltage-current characteristic of an organic EL element is a steep-slope characteristic. Therefore, in the case of the constant-voltage driving, even slight variation in the voltage and element characteristics causes large variation in the current, resulting in large variation in the luminance. Consequently, the constant-current driving in which the drive transistor is used in its saturation region is generally employed. Although current variation causes luminance variation also in the constant-current driving of course, small current variation causes only small luminance variation.
Conversely, even in the constant-current driving system, it is important that a drive signal written and held in the holding capacitor depending on an input image signal be constant in order to ensure invariant light emission luminance of an electro-optical element. For example, in order to ensure invariant light emission luminance of an organic EL element, it is important that the drive current dependent upon an input image signal be constant.
However, variation in the threshold voltage and the mobility of the active element (drive transistor) for driving an electro-optical element is caused by variation in the process for the active element. Furthermore, characteristics of the electro-optical element such as an organic EL element change over time. If there are such variation in characteristics of the drive active element and change in characteristics of the electro-optical element, the light emission luminance is adversely affected even in the constant-current driving system.
To address this problem and achieve uniform light emission luminance across the entire screen of a display, various studies have been made about schemes for correcting luminance variation attributed to the above-described variation in characteristics of the drive active element and the electro-optical element in each pixel circuit.
For example, in Japanese Patent Laid-open No. 2006-215213 (Patent Document 1), the following functions have been proposed as functions of a pixel circuit for an organic EL element; a threshold correction function for keeping a drive current constant even when the threshold voltage of the drive transistor involves variation and change over time; a mobility correction function for keeping a drive current constant even when the mobility of the drive transistor involves variation and change over time; and a bootstrap function for keeping a drive current constant even when the current-voltage characteristic of the organic EL element involves change over time.
However, the scheme described in Patent Document 1 requires, as additional components, interconnects for supplying potentials for the correction, two switching transistors for the correction, and two kinds of switching pulses for driving these switching transistors. As a result, this scheme has a 5TR-drive configuration employing five transistors, including a drive transistor and sampling transistor, and hence the configuration of the pixel circuit is complicated. Because the number of components in the pixel circuit is large, enhancement in the definition of the display is precluded. As a result, it is difficult for the 5TR-drive configuration to be applied to a display used in a small electronic apparatus such as a portable apparatus (mobile apparatus).
Therefore, there is a need for the development of a system that suppresses luminance variation due to variation in element characteristics with a simplified pixel circuit. This development should be so made that a new problem that is not involved by the 5TR-drive configuration but accompanies the simplification will not occur.